One of the preferred uses of the invention is in the field of isomerizing gasoline so as to improve the octane number thereof. From the octane number standpoint, it is desirable for the hydrocarbons constituting the gasoline to be as branched as possible. For example, dimethyl butanes have a higher octane number than methyl pentanes. In order to increase the level of polymethyl branched paraffins in the charge from the isomerization zone, it is possible to use molecular sieves selective with respect to the size of the accessible pores and recycle to the isomerization process both the normal paraffins and the monobranched paraffins. Thus, the process according to the invention permitting the at least partial separation of the paraffins from the isomerization process, optionally combined with a recycling to the isomerization process of normal and/or monomethyl branched paraffins, makes it possible to improve the gasoline octane number.
The previously described processes involve the partial or total isomerization of normal paraffins. These processes described in U.S. Pat. No. 4,210,771 (Holcombe), U.S. Pat. No. 3,755,144 (Asselin) and U.S. Pat. No. 4,476,345 (Gray), use zeolites which, due to the diameter of their pores, adsorb the normal paraffins, but not the branched or cyclic products. The most frequently used zeolite is the 5A zeolite, which is a A zeolite, whose substitution cation is calcium, the diameter of its pores being between 3 and 5 .ANG.. It adsorbs normal paraffins, but it does not absorb branched paraffins, not even monomethyl branched paraffin.
U.S. Pat. No. 4,804,802 leads to an improvement to the extent that it describes the use of another molecular sieve permitting the recycling to the isomerization process not only of normal paraffins, but also monomethyl branched paraffins.
The principle of U.S. Pat. No. 4,804,802 is to treat the isomerized hydrocarbon fraction on a molecular sieve bed, whose pores have a diameter equal to or smaller than 4.5 .ANG. and which only adsorbs normal paraffins, allowing the monomethyl branched and obviously the more branched paraffins to percolate. The second molecular sieve bed is constituted by a zeolite, whose pore diameter is between 4.5 and 5.5 .ANG.. This bed adsorbs the monomethyl branched paraffins, whereas the more branched paraffins are not retained and constitute the isomerized fraction with the improved octane number. The first molecular sieve bed described in this patent can be a 5A sieve substituted with calcium, but it is also possible to use other molecular sieves, such as R or T zeolites or even natural zeolites such as chabazite or erionite.
The molecular sieve of the second bed described in the above patent must have a pore size intermediate between that of the calcium 5A zeolite and that of the ZSM5 zeolite, i.e. the pore size is between 4.5 .ANG..times.4.5 .ANG. and 5.5 .ANG..times.5.5 .ANG.. The zeolite recommended in U.S. Pat. No. 4,804,802 is ferrierite, which e.g. adsorbs monomethyl pentanes. Ferrierite can be used in hydrogen form, but preference is generally given to the form substituted by an alkali metal, an alkaline earth metal or a transition metal. Other adsorbents can be used and are claimed in the above patent and are in particular constituted by aluminophosphates, silicoaluminophosphates and borosilicares. All these adsorbents retain monomethyl branched paraffins and exclude dimethyl branched and polymethyl branched paraffins.
Ferrierite and also the other adsorbents with a pore size between 4.5 .ANG..times.4.5 .ANG. and 5.5 .ANG..times.5.5 .ANG. have two major disadvantages, namely the small diameter of the pores, necessary for obtaining a selectivity, which necessarily limits the adsorption capacity, as well as the adsorption and desorption kinetics, and the diameter of the pores, which is fixed once and for all for a given adsorbent type, is not of an optimum nature and does not permit a total separation between the monomethyl branched and dimethyl branched paraffin hydrocarbons. Moreover, in the case of ferrierite, it is virtually obligatory from a technical standpoint to use a synthetic ferrierite having far fewer structural defects than natural ferrierite.